Rubber modified styrene-based resins represented by high impact polystyrene (HIPS) are excellent in impact resistance, moldability, and dimensional stability, and thus such resin is widely used as a food packaging material, such as deli containers, lunch box containers, various trays, beverage containers, and food container lid members.
However, as well known, such rubber modified styrene-based resin is not resistant to environmental stress cracking by oil and chemicals and thus may crack in a short time due to the contact with fat and oil contained in foods depending on the environment of use. In particular, when the rubber modified styrene-based resin is used as a food container lid member, such as a coffee cup lid, there has been a problem of accelerating cracking in the container by the stress for repeated fitting.
As a method of improving the oil resistance of such rubber modified styrene-based resin, there is a method in which a polyolefin-based resin film is laminated on a rubber modified styrene-based resin sheet. This method, however, has problems of not only a great increase in processing costs but also non-recyclability of trimmed scrap produced when a food container is molded by thermoforming, such as pressure forming and vacuum forming, them.
As other methods of improving oil resistance, PLTs 1 through 3 disclose methods in which a dispersed particle diameter of a rubber modified styrene-based resin is increased. Further, in order to improve fitting properties of a food container, PLT 4 discloses a method in which a dispersed particle diameter, a gel component, and a graft ratio of a rubber modified styrene-based resin are defined in specific ranges.
These methods, however, are not concerned with folding endurance and tear strength when a rubber modified styrene-based resin sheet is used as a food container and thus are insufficient to cope with cracking of the food container lid member for repeated fitting.
As a method of improving folding endurance and tear strength of a rubber modified styrene-based resin sheet, there is a method in which a styrene-butadiene copolymer is added. The copolymer has to be added in a large amount to give oil resistance, and as a result, rigidity and fitting properties as a food container and a food container lid member are reduced. In addition, there has been a problem of deteriorating the appearance of a sheet due to foreign gel objects by a crosslinking reaction of the styrene-butadiene copolymer during molding process.